Polybutylene terephthalate resin is used in a wide range of fields as an engineering plastic, such as automobile parts, electric/electronic parts due to the excellent mechanical properties, electric properties, and other physical and chemical properties, along with good workability. In particular, in order to protect the contents from damages caused by moisture, dust, external shock, and the like, polybutylene terephthalate resin is preferably used for the casing containing base mounted with electronics parts thereon, and the insert-molded products on which sensor probe, connector terminal, and the like are mounted, and the like.
In these applications, for the purpose of avoiding as much as possible, the influence of moisture, dust, and the like on the base and the sensor body, a potting material is often heat cured by being filled in a cover with a base stored therein or a cover and a casing are often bonded and sealed with an adhesive. Recently, various joining methods such as double shot molding, hot-plate welding, vibration welding, and laser welding have been put into practical use. As for the potting intended for protecting the base, epoxy resins and silicone rubber are widely used independent of the joining methods and in particular, silicone rubber is often used for components requiring heat resistance and cold resistance.
In the application where the above methods are used, there are normally arranged metal terminals such as connector, metallic bus bar constituting electric circuit, varieties of sensor parts, and the like by pressure-working or insert-molding. Specifically in applications for parts mounted on automobile, high durability is required in many cases in an environment of high temperature/high humidity and of heating and cooling cycles. Accordingly, there are commonly used polybutylene terephthalate materials characterized by elastomers and various additives.
In consideration of such demands, in JP-A 63-3055, a molded product is proposed which is formed of polybutylene terephthalate and specific acrylic-based rubber and which is obtained by insert-molding a metal or an inorganic solid.
Although any of the materials proposed in JP-A 63-3055 exhibits superior characteristics to a certain extent regarding the heating and cooling cycle properties of the insert-molded product, the adhesion properties to silicone rubber are not mentioned and any of them actually shows seriously poor performances, and a surface for adhesion is preferably worked by an etching process and the like for adhesion, thus an alternative welding technique having been strongly desired.
Regarding adhesion with silicone rubber, the adhesion properties may be inhibited due to, not only the influence of a physical surface state (roughness) of the adherend material, but also the influence of chemical composition, for example, bleeding of additives such as lubricants, and chemical reactions of the adhesive may be suppressed due to the additives, and thus conventionally the composition of the adherend material has been prone to be a factor that determines the adhesion strength.
In parts for holding electronic parts requiring heat resistance, for the purpose of protecting the electronic parts from moisture and chemical substances, it is essential to inject silicone rubber into a casing for sealing after holding the electronic parts, and depending on the selection of the material of casing and cover, there were cases where that kind of silicone rubber caused poor curing and poor adhesion at the interface, and thus was unable to exert sufficient function as the parts, in some cases.
Although various known welding methods are proposed as a method of joining a casing for holding such electronic parts, joining with silicone rubber is usually carried out to avoid damages on the electronic parts in spite of the occasional occurrence of a curing failure or an interfacial adhesion failure as described above not to be able to function sufficiently as parts.
Further, JP-A 9-165503 and JP-A 10-316844 propose a composition containing, a specified polybutylene terephthalate, a specified silicon compound, a phenol-based antioxidant and/or a thioether-based antioxidant in order to improve the adhesion strength to an addition-reaction type silicone.
That kind of composition, however, cannot satisfy the heating and cooling cycle resistance of the material required in the automobile industry. Generally it is known that the addition of a specific silicon compound in large amounts deteriorates the physical properties of silicone rubber, and the presence of sulfur-based compound and trivalent phosphorus compound deactivates the platinum compound in the curing catalyst to thereby inhibit the reaction. Therefore, compositions disclosed in JP-A9-165503 and JP-A10-316844 are not practically applicable.
Further, in JP-A 2007-91842, although a use of an acrylic-based elastomer having a glycidyl group is proposed as a technique to improve the adhesion properties to epoxy or addition-type silicone rubber, the viscosity of the polybutylene terephthalate resin composition rises, so that the flowability may decrease.